Why can I not go into pose mode?
To switch to Pose Mode you just need to select the armature object and make it the active object (=highlighted in orange). It's called RootNode in the outliner. You have selected it but additionally, you have selected the mesh Genesis2Female-skininstance and this mesh is the active object. That's the reason why you can't switch to Pose Mode.
The deformation of the mesh in the pose mode works but the bones are all very tiny and they all stand vertically upwards. If the model was imported as an FBX file you can adjust the import settings for the bones. See this question: Rotate all bones in rig by 90 degrees
Edit: You wrote that it was imported as collada file. But there are also options for the bones in this case. Try Fix Leaf Bones.